Romans 7:21-23 meaning

Paul values and agrees with the Law and wants to do good. Therefore, he says I find then the principle that evil is present in me, the one who wants to do good (v 21). However, Paul's sin nature (which, Paul tells us, all humans have) wants to disobey the Law. Paul points out that while he wants to follow the Law of God and joyfully concurs with the Law of God in the inner man (v 22), I see a different Law in the members of my body, waging war against the Law of my mind and making me a prisoner of the Law of sin which is in my members (v 23). 

Paul here elevates a mental model of the inner-self for any believer. We have three actors:

• A flesh, which Paul here refers to as a Law in the members of my body. This is our old nature, the one we are born with. Paul asserts that there is nothing good in this nature (Romans 7:18). This means the only positive thing we can do with this nature is to recognize it, and avoid it. There is nothing good that can come from walking in our flesh. This is made clear in the previous passage, Romans 7:15-20.
• A redeemed spirit, in which dwells the mind of Christ (Romans 8:2, 2 Corinthians 5:17, Galatians 5:17).
• A will, that chooses whether to walk in the flesh, or the Spirit (Romans 8:4, Galatians 5:13).

In this passage Paul refers to the Law of my mind that agrees with God's Law. By general application, this could refer to the fact that each person is made in God's image, and therefore instinctively knows what is true (Genesis 1:27, Romans 1:19-20, 2:14-15). Also, since Paul is a believer, it could refer to the fact that he is indwelt by the Holy Spirit, who speaks and leads us into the truth (Romans 8:14).

The flesh holds Paul, he says it is making me a prisoner of the Law of sin which is in my members. Paul feels he is caught in the gravity of the Law of sin. In his own strength he cannot escape. This passage expands on verse 18, where Paul says "nothing good dwells in me, that is, in my flesh." The flesh, the old nature, is un-reformable.

When we walk in the flesh, our old nature, we can't help but walk in sin. Even though it is not what we really want, and we know it is wrong. We are trapped. We need to be delivered. In Romans 6:12, Paul tells us that sin can take control (reign) in our mortal bodies. However, Paul asserts that believers should not let this happen because we are under grace and not the Law.

Thankfully, Paul will tell us that deliverance is ours, when we choose to submit to the Spirit. But this does not come through reforming the flesh. It does not come through obeying religious rules, like being circumcised and following Jewish religious practices. Deliverance from the flesh does not come through willpower. Rather, deliverance from the flesh comes through a walk of faith (Romans 1:16-17). We gain deliverance from the ongoing power of our old nature, the flesh, through separating ourselves from our flesh (putting it to death) and walking in the resurrection power of Jesus (Romans 7:25 - 8:4).